Large steam turbines generally include multiple nozzle chambers through which steam is directed into the turbine and through turbine blades which are rotated thereby. Nozzle chamber activation (steam admission thereinto) is regulated by valves which open to provide steam flow from steam supply conduits into the nozzle chambers and close to obstruct steam flow thereinto. A valve point is defined as a state of steam admission in which each valve is in the completely open, unobstructing configuration or the completely closed, full obstructing configuration. It can be shown that maximum turbine efficiency can be attained from the use of an infinite number of valve points which, in turn, requires an infinite number of valves.
Of course, a finite number of valves must be used on steam turbines with that number of valves being dictated by compromises between improved turbine performance and increasing capital cost for increasing numbers of valves. One or more valves control the flow of steam into each nozzle chamber. Nozzle chamber activation refers to the process of increasing steam flow into the nozzle chambers from the time steam flow thereinto is initiated until the maximum steam flow thereinto (completely activated) is achieved. Deactivation refers to the process of decreasing steam flow into the nozzle chambers. When multiple valves are used to regulate steam flow into a single nozzle chamber, those valves typically modulate together. Since such valves modulate together, turbine efficiency is actually a maximum when the nozzle chambers are each in the completely activated or completely deactivated state.
Heretofore, the nozzle chambers were activated in a predetermined sequence such that once the nozzle chamber was activated during increasing load on the turbine, it was not deactivated until the load on the turbine decreased. One of the few restraints on nozzle chamber activation sequence was that single shock operation was preferred over double or multiple shock operation. That is, it is usually preferable practice to activate nozzle chambers such that each newly activated nozzle chamber (after minimum admission) is circumferentially adjacent at least one previously activated nozzle chamber.